1. Field of the Invention
The invention relates to a multi-part cooled piston for an internal combustion engine, comprising a one-piece piston upper part having a combustion bowl and an annular wall with piston-ring part, and a one-piece piston lower part having a box-like piston skirt, and bosses to receive the piston pins joining the piston to the connecting rod and boss supports, which are joined to the piston skirt. A cooling channel is formed through the piston upper part and piston lower part and is limited thereby in its cross section.
2. The Prior Art
Such pistons are known, for example, from German Patent No. 800350, German Unexamined Application No. 2212922, European Patent No. 0604223 A1, Japanese Patent No. 60-143148 and Japanese Patent No. 60-178345. These pistons are provided with substantially central screw couplings and thus have the known disadvantages of what are known as built pistons, in which, due to the point connection of the piston upper and lower parts, a relative motion causing wear is developed between two piston components under the combustion pressure. In order to achieve the advantage of these pistons that results from the connection of wear-resistant piston upper parts with piston lower parts made of light metal, without suffering the aforesaid disadvantages, the screw couplings have been shifted to the outer rim region of the piston, as is known, for example, from French Patent No. 753615 or U.S. Pat. No. 2,159,989. In general, these designs, which date from the 1930s, are not comparable with the concepts of modern pistons, since they were designed for much lower combustion-chamber pressures. In addition, it was not possible to embody combustion bowls in such designs.
East German Patent No. 123962 shows a piston in which a wear-resistant piston-ring area is created by mechanically joiningxe2x80x94by screw coupling in this casexe2x80x94a piston base body containing a fire ring to the piston-ring area or to a part of the annular element forming part of the piston-ring area, and securing it against loosening. A disadvantage, however, is that only the piston-ring area is made of wear-resistant material, and high compression pressures cannot be achieved in combination with low overall piston height.
Besides the central or off-centered screw coupling as the joint between piston parts made of different materials, there are also known welding methods, such as the friction welding method of International Patent WO 00/06882. Therewith the piston upper and lower parts can be welded together relatively simply. This has the disadvantage in that welding changes material microstructure, thus leading to stress cracking. An even greater disadvantage, however, is the limitation on choice of materials for the parts to be joined.
It is therefore an object of the invention to provide a multi-part cooled piston for an internal combustion engine which permits a reduction of the compression height and an increase of the heat load despite increasing engine power, and which also permits assembly of the piston without microstructural change.
This object is accomplished by a piston comprising a piston upper part having a combustion bowl and an annular wall with piston-ring part, and a piston lower part having a box-like piston skirt, bosses to receive the piston pins joining the piston to the connecting rod and boss supports, which are joined to the piston skirt, and a cooling channel formed through the piston upper part and lower part and being limited thereby in its cross section.
According to this solution, the piston upper part and the piston lower part are provided with support elements having seating faces, which form a first and a second seat. The support element of the piston upper part and lower part comprising the first seat is provided with threads for screwing the two piston parts together. In the screwed together condition of the piston upper part and piston lower part, the two piston parts are supported exclusively via the first and second seats. The mechanical stability achieved thereby is superior to that of a welded joint between the piston parts, because of the absence of internal stresses. In particular, this stability is achieved by the arrangement of the seats relative to the piston diameter in such a way that the first seat is disposed radially inward and the second radially outward. These seats are disposed in different planes relative to the piston height, so that effective internal and external bracing is achieved.
According to the invention, one part of the support elements, which are constructed as an annular rib and an annular bearing rib, is provided with a thread for screw coupling of the piston parts, the wall structure of the annular rib being configured in such a way that elastic deformation in response to the combustion-chamber pressure exerted on the piston bottom can take place without diminishing the stability of the screw coupling. In particular, because of the positional arrangement of the support elements, which are defined by the radii that extend from the longitudinal axis of the piston in a radial direction to the outer surface of the piston, the flow of force from the piston bottom via the support elements to the boss supports or piston pins is effectively dissipated without overloading the piston material. Damage due to mechanical load, such as stress cracking, can therefore be effectively prevented.
Furthermore, the inventive design has the advantage that the centering of the piston upper part relative to the piston lower part takes place not by the screw coupling but by wall regions of a step-like structure of the support element that forms the second and therefore outer seat. Therefore, piston upper parts of heat-resisting steel and piston lower parts made of forged AFP steel can be joined particularly simply and inexpensively.